He led Labour to its worst-ever defeat, and a Massey University expert says he has grounds to feel he was unfairly treated by four of the country's biggest newspapers.

Associate Professor Claire Robinson has assessed all the images run in those papers in the last month of the campaign, and found John Key's picture featured 138 times while Mr Goff featured only 80 times.

"It would have substantially helped to have had favourable coverage and greater coverage, and particularly of photos," said Mr Goff.

Mr Goff wants the papers to make a commitment in future election campaigns to provide balanced coverage.

The papers looked at were the New Zealand Herald, Dominion Post, Sunday Star-Times and Herald on Sunday.